Life cycle assessment of anaerobic digestate valorization through hydrothermal carbonization

Abstract

Anaerobic digestion has been used in rural Bangladesh for several decades to manage livestock and agricultural waste, resulting in biogas production. Despite the benefits associated with biogas generation, the effective management of residual biogenic material or digestate remains challenging. The current practice of handling digestate through open dumping can cause severe environmental issues such as nutrient imbalance and greenhouse gas emissions. Hydrothermal Carbonization (HTC) has emerged as a promising technology for valorizing anaerobic digestate by converting it into valuable products. This study aimed to compare the environmental impacts of anaerobic digestate’s HTC with traditional open dumping from a life cycle perspective. A gate-to-gate life cycle assessment using the ILCD 2011 Midpoint+ method showed that the HTC scenario outperforms open dumping in six of the nine impact categories, specifically reducing acidification, climate change, freshwater ecotoxicity, freshwater eutrophication, human toxicity, and marine eutrophication. For a functional unit (FU) of 1 tonne of anaerobic digestate, HTC reduces acidification and climate change impacts by 78% and 61%, respectively. Additionally, the HTC scenario has 0.4634 kg P eq/FU less impact on freshwater eutrophication than open dumping and reduces freshwater ecotoxicity and particulate matter formation by 93% and 27%, respectively. However, the HTC of anaerobic digestate increases marine eutrophication, photochemical ozone formation, and water resource depletion. Proper post-processing of HTC liquid products and an increase in the energy efficiency of the process can address these issues. Uncertainty analyses via Monte Carlo simulations supported the findings favoring the HTC scenario. Hotspot analysis revealed that the HTC reactor is the main contributor to the most impact categories. Implementing an energy recovery system from the HTC flue gas and utilizing renewable energy sources to meet the energy requirements of the HTC reactor can significantly improve the overall sustainability of the process.